Untreated water will have a distinct H2S odour about 3 months after annual well chlorine shock treatment.

Current system is a Star water systems Ultra Pure 11 filter, this uses air injection followed by a 100 gallon air over water contact tank before the filter unit which contains a Birm, Calcite mixture. Claimed to remove iron, manganese and H2S. It doesn't work!!!! I notice Star no longer sell this system. The contact tank is an old galvanized tank and will be replaced whichever option below is chosen.

Following the star filter is a conventional softener and finally a UV sterilization step. Softener works fine. Drinking water is by RO unit on the softened water stream, no problems with this.

House has 3 full bathrooms and 3 half baths but 95% of occupancy is only 2 people.

Maintenance is normal replacement of RO fiters and membranes, salt replenishment, annual well and whole system shock chlorination.

PROBLEM: Occasional slight H2S odour from none RO softened cold water. occurs about 3 months after shock chloination. I'm convinced the H2S comes from iron or sulphate bacteria which re-populate the well over time following shock chlorination and which the Star fiter system, Birm active ingredient, will not remove.

Solution alternatives currently under consideration.

Option 1 the lower cost likely workable

Remove the star filter, the air injector and the old contact tank and install a Greensand filter with Pot. Perm regeneration, then continue to existing softener etc.

Option 2 the highest cost, Bullet proof??

Remove the star filter, the air injector and the old contact tank and install a proportional chlorinator followed by a new 120 gallon contactor tank, followed by a nexsand filter then a carbon filter then on to the softener etc. Possible use of a magox inplace of the nexsand filter.

Comments on the above would be appreciated and any other alternatives gratefully received.

I would not go with greensand filter. You will understand when you are in the shower and your water turns purple. Besides, look at the cost of the pot-perm itself. I also would not shock the well. In 23+ years in water treatment, I have heard the horror stories from shocking the well. The best solution is the chlorination/dechlorination. I have found that the media used to remove iron and sulfur is very heavy and most wells do not have the flow rate to properly backwash the media. If that is the case with your well, you will be in the same boat you are now in a short time. I don't see a need the remove the current retention tank or the nexsand filter. The carbon will remove the oxidized h2s and the chlorine.

Recalling that chlorine can rust steel well casing I'd go with as little as possible, I'd continue doing as you have been but replace the Birm and calcite with Centaur carbon, H2S ruins Birm. And if you must, replace the galvanized retention tank with the mixing/retention tank at this link.